The invention relates to a novel, vanadium-free catalyst for the selective catalytic reduction of nitrogen oxides with ammonia or a compound that decomposes to ammonia as reducing agent, which is especially suitable for removing nitrogen oxides from exhaust gases from primarily lean-burn internal combustion engines in motor vehicles. The invention relates further to a method of activation of a homogeneous cerium-zirconium mixed oxide for the selective catalytic reduction of nitrogen oxides.
The emissions of a motor vehicle can basically be divided into two groups. Thus, the term raw emission denotes pollutant gases that are formed directly in the engine through the fuel combustion process and are already contained in the exhaust gas before passing through the exhaust gas cleaning system. Exhaust gas components that may form as by-products in the exhaust gas cleaning system are termed secondary emissions.
Exhaust gases from motor vehicles with a primarily lean-burn internal combustion engine contain, in addition to the usual primary emissions carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx, a relatively high oxygen content of up to 15 vol. %. Carbon monoxide and hydrocarbons can easily be made harmless by oxidation. However, reduction of the nitrogen oxides to nitrogen is much more difficult, on account of the high oxygen content.
A known method for the removal of nitrogen oxides from exhaust gases in the presence of oxygen is the method of selective catalytic reduction (SCR process) by means of ammonia on a suitable catalyst, called an SCR catalyst for short. In this method, the nitrogen oxides that are to be removed from the exhaust gas are reacted with ammonia to nitrogen and water. The ammonia that is used as reducing agent can be made available by feeding a compound that decomposes to ammonia, for example urea, ammonium carbamate or ammonium formate, into the exhaust system and subsequent hydrolysis. Furthermore, exhaust gas cleaning concepts for motor vehicles are known in which ammonia is produced as secondary emission on a series-installed catalyst during a rich operating phase of the engine, for example a nitrogen oxide storage catalyst, and is stored temporarily in the SCR catalyst until the time of consumption during the lean operating phases.
Use in a discontinuously operating SCR system, in which the ammonia used as reducing agent is produced exclusively as secondary emission in the exhaust system, requires SCR catalysts with sufficient ammonia storage capacity to provide if at all possible the entire requirement for the reducing agent for removal of nitrogen from the exhaust gas during the lean phase. Zeolite-based SCR catalysts, as described in numerous publications and patent applications, are particularly suitable for this. For example, U.S. Pat. No. 4,961,917 describes a method for reduction of nitrogen oxides with ammonia using a catalyst which, in addition to a zeolite with defined properties, contains iron and/or copper as promoter. Other SCR catalysts based on transition-metal-exchanged zeolites and methods of selective catalytic reduction using said SCR catalysts are described for example in EP 1 495 804 A1, U.S. Pat. No. 6,914,026 B2 or EP 1 147 801 B1.
For use in exhaust gas cleaning systems with metering devices for urea or other compounds that decompose to ammonia, providing continuous supply of reducing agent to the system, it is not necessary for the SCR catalyst to have a large ammonia storage capacity. Accordingly we try to avoid using zeolite-based SCR catalysts, as they are very expensive on account of the high production costs for zeolite compounds.
SCR catalysts that contain vanadium oxide in addition to titanium dioxide or tungsten oxide or mixtures thereof are suitable for this. For example, EP 0 385 164 B1 describes such a catalyst that contains, in addition to titanium dioxide, at least one oxide of tungsten, silicon, boron, aluminium, phosphorus, zirconium, barium, yttrium, lanthanum or cerium, and at least one oxide of vanadium, niobium, molybdenum, iron or copper and that is produced as a moulded article by compaction or extrusion of the components, optionally after adding suitable auxiliary agents. EP 1 153 648 A1 describes a structured SCR catalyst, which contains, beneath a coating of hydrolysis catalyst, a reduction coating whose composition corresponds to the recipe known from EP 0 385 164 B1. EP 0 246 859 describes an SCR catalyst that contains vanadium, applied on a mixture of cerium oxide and aluminium oxide.
An important problem when using vanadium-containing SCR catalysts for cleaning exhaust gases from motor vehicles is the possible emission of volatile, toxic vanadium compounds at higher exhaust gas temperatures, probably with harmful effects on humans and the environment. Accordingly there is low market acceptance of vanadium-containing car exhaust catalysts.
Efforts have already been made since long ago to provide vanadium-free SCR catalysts as a cost-effective alternative to high-priced, zeolite-based systems.
For example, U.S. Pat. No. 4,798,817 describes an SCR catalyst that essentially contains from 0.5 to 50% iron sulphate applied on a mixture of 2 to 60% cerium oxide and aluminium oxide. U.S. Pat. No. 4,780,445 describes an SCR catalyst with 0.1 to 25% nickel sulphate or manganese sulphate or mixtures thereof applied on a mixture of 2 to 60% cerium oxide and aluminium oxide.
JP 2005-238195 or EP 1 736 232 describes a catalyst system for the removal of nitrogen oxides, containing a first reaction section for denitration by reaction of nitrogen oxides with ammonia and a second reaction section for oxidation of excess ammonia, in which the first reaction section contains a first catalyst, which contains as active component at least one complex oxide containing two or more oxides selected from the group comprising silicon oxide, aluminium oxide, titanium oxide, zirconium oxide, and tungsten oxide, and a rare-earth metal or transition metal except Cu, Co, Ni, Mn, Cr and V.
Apostolescu et al. describe, in Appl. Catal. B: Environmental 62 (2006) 104, investigations on SCR powder catalysts of Fe2O3/ZrO2 with WO3 doping in synthetic model exhaust gas.
The known vanadium-free and zeolite-free SCR catalysts are sometimes of complicated composition, difficult to manufacture and/or do not satisfy the more stringent requirements on activity and ageing stability for use in motor vehicles.
The problem to be solved by the present invention is therefore to provide an SCR catalyst that is vanadium-free, and can be manufactured more cheaply than zeolite-based systems and with simple means. In comparison with existing known systems the catalyst should have a high catalytic activity and good ageing stability, and it should be suitable in particular for the removal of nitrogen oxides from the exhaust gas of primarily lean-burn internal combustion engines in motor vehicles with a continuous SCR system including a metering device for ammonia or a compound that decomposes to ammonia.